The invention relates to infra-red scanning microscopy and has particular application in the assessment of semiconductor wafers and slabs.
Modern electronic devices are fabricated on electronic grade semiconductor wafers. These are cut from ingots grown from the melt, generally by the Czochralski method. The materials used may be silicon, Group III-V materials such as Gallium-Arsenide or Group II-VI materials such as Cadmium-Telluride.
It is important to be able to assess the quality of the wafers (or slabs) since dislocations occur and there are local precipitations of impurities which may affect the behavior of the material in subsequent heat treatment and which may affect the performance of the electronic devices. A particular problem concerns silicon. Czochralski silicon ingots contain oxygen which has disadvantages and advantages. The principal disadvantage is that in the heat treatment processes associated with the electronic device fabrication, oxygen precipitates out as oxide particles. However, although basically an impurity, the oxide can be used to advantage by careful heat treatment. This is because the oxide particles perform a gettering action on other impurities such as metallic copper. Therefore if, by an annealing process, the oxide particles can be encouraged to precipitate away from a surface on which the electronic devices are to be fabricated, they can be used to draw away other impurities which may be introduced in the fabrication process. Another advantage of the oxygen is that it rigidifies the material.
Thus, the presence of oxide particles can be an advantage in silicon but their number density, size and distribution must be accurately controlled and must therefore be accurately assessed in a given semiconductor wafer. A semiconductor wafer may be 4 to 8 inches in diameter and 0.5 to 1.0 mm in thickness. One surface is polished flat to accept the electronic devices, the other surface is usually `wavy`, and the edge is bevelled. The assessment of wafers is generally by optical microscopy. The wafer is broken and the exposed edge is etched and microscopically examined. This method has the disadvantage that only the exposed surface can be examined and a three-dimensional assessment is not readily possible.
More recently, infra-red imaging techniques have been developed. Semiconductor wafers are transparent to infra-red light. An optimum wavelength for silicon is about 1.3 .mu.m. The light may be produced by a solid state, gas or semiconductor laser and may be focussed to a narrow beam which passes through the wafer. The axis of the laser beam is generally perpendicular to the surface of the wafer. By effecting a scanning movement of either the beam or the wafer in a raster and synchronously interpreting the output of an infra-red detector directed at the wafer, an image can be built up for display on a cathode - ray tube or computer display. Image processing techniques can be used to improve spatial and depth resolution and contrast and alleviate the effects of noise.
Infra-red scanning microscopy can be used in two ways: (a) bright field, or transmission, mode in which the detector looks back along the beam axis and particles in the wafer scatter the light and appear as dark spots against a bright background and (b) dark field, or scattering, mode in which the detector looks obliquely at the wafer and receives light scattered from particles which appear as bright spots against a dark background. Individual particles down to about 30 nm in diameter can be imaged. These techniques have the advantage of being non-destructive of the wafer.
For silicon wafers of these thicknesses, the quality of the image is degraded if the light used to form the image either enters or exits the wafer through a non-flat surface. For bright-field, or for dark-field with the scattering angle less than 90.degree., high quality images are obtained only if both surfaces of the wafer are polished flat.
A variation of the foregoing technique is to arrange the infra-red detector view the infra-red light at exactly 90.degree. to the incident beam. This is achieved by breaking the wafer and polishing flat the exposed edge at 90.degree. to the polished wafer surface. This technique can produce high quality images. However, it has the disadvantage, along with the etching microscopy method, that the wafer is destroyed in the process of assessment. All that can be assessed, therefore, are wafers which are hopefully similar to those which are to be used in fabrication. It would be an advantage to provide a truly non-destructive method of three-dimensional assessment with satisfactorily high resolution, sensitivity and signal-to-noise ratio. The present invention seeks to provide such a method and a microscope which employs it.